Such a charged particle source is known from U.S. Pat. No. 7,034,315, which is assigned to the assignee of the present invention and which is hereby incorporated by reference. The charged particle beam described therein can be used as e.g. an electron source in an electron microscope, such as a Scanning Electron Microscope (SEM), a Transmission Electron Microscope (TEM) or as an ion source in a Focused Ion Beam (FIB) apparatus.
In an apparatus using a charged particle source the beam is manipulated by particle-optical elements, such as lenses and deflectors. The beam is manipulated to e.g. form a focus on a sample and scan it over the sample.
The size of such a focus on a sample is at least in part determined by the so-named chromatic aberrations. Chromatic aberrations in particle-optical instruments are those aberrations that are caused by the particles in the beam having slightly different energies. For an electron source used in an electron microscope the energy spread of the electrons is typically in the order of between 0.3 to 1 eV, for an ion source used in a FIB the energy spread is typically between 1 and 10 eV. Especially when using low beam energies the relative energy spread ΔE/E is large, resulting in chromatic aberrations.
It is remarked that the chromatic aberrations can be reduced by reducing the beam diameter, but this results in lower beam current and may increase the contribution of the so-named diffraction to the spot size.
It is further remarked that also charged particle apparatus are known in which the sample is not illuminated with a focused beam, but with a parallel beam. In this case as well the chromatic aberrations are important.
The charged particle source described in U.S. Pat. No. 7,034,315 provides a solution to the problem of chromatic aberrations by offering a source with reduced energy spread. This is accomplished by filtering a part of the charged particles showing a relative small energy spread and stopping the rest of the particles. The eccentric beam will, due to the energy dispersive working of the lens, be imaged as an energy dispersed line on the energy selecting diaphragm. The width of the energy selecting aperture in the diaphragm on which it is imaged determines the energy spread of the energy selected beam passing through said aperture.
The known filter uses an off-axis part of a lens as energy dispersive element and forms an image of the particle emitting surface on an energy selective diaphragm. The energy dispersive working of the lens forms a dispersion line on a small aperture in the form of a slit in the diaphragm, passing a part of the energy disperse image and blocking the rest.
A problem of this energy filter is that at least two beams leave the gun module: the central beam and the energy filtered beam. Typically one of these is centred around the axis while the other diverges from the axis. This diverging beam may cause unwanted reflections, contaminations etc.
Another problem is that the charged particles of the energy filtered beam interact with the particles in the central beam. During this interaction the energy spread of the energy filtered beam may increase due to Boersch effect and trajectory displacement.